EL PASO — Federal authorities are treating the death of a U.S. Border Patrol agent as a potential assault on a federal official, but they have not ruled out the possibility that Agent Rogelio Martinez died in an accident on a moonless night in rough terrain.

An FBI investigation is underway to determine the cause of Martinez’s death late Saturday along a desolate span of Interstate 10 in West Texas, Special Agent in Charge Emmerson Buie Jr. said Tuesday at a news conference.

President Donald Trump and union officials have publicly stated that Martinez was attacked, but Buie cautioned against rushing to judgment.

“Our presence and our investigation is to do everything in our power to identify the facts as they are presented,” he said. “Right now, we’re investigating this matter as a potential assault on a federal officer and we’re trying to gather the facts, and if facts support that the incident was caused by something else, we will present that.”

Martinez and the other agent were "responding to activity" outside Van Horn on I-10, about 120 miles east of El Paso.

Officials haven't eliminated the possibility that Martinez, who was found unconscious with broken bones, and his partner, who suffered head trauma, were injured in some other way, including falling from a culvert.

Border Patrol union officials say the surviving agent has no recollection of what happened.

“We’re looking at all possibilities,” said FBI Jeanette Harper.

Speculation about what might have happened includes suggestions that the agents were attacked by immigrants or smugglers. That prompted some to immediately call for more resources on the border.

The president tweeted that those responsible for Martinez’s death would be held accountable and renewed his call for a border wall. Texas Gov. Greg Abbott announced an award of $20,000 for information leading to the arrest of anyone involved in what he called a “murder.”

Leaders with the National Border Patrol Council, the union representing agents, said they received information from agents on the ground that the two agents were attacked with rocks after stopping on the side of interstate.

“All our evidence is consistent that this was the work of smugglers,” said Art del Cueto, the union’s vice president. “We’re talking about the border and in a part of Texas ripe with drug smugglers.”

Martinez, 36, a native of El Paso and a four-year veteran, is survived by his parents, brothers, a son and fiancé, said Victor Velazquez, acting chief patrol agent for the Big Bend Sector. The other agent, who has yet to be named, is in stable condition in intensive care at an El Paso hospital.

“It’s always hard when you lose a young and dedicated agent,” Velazquez said.

He said Martinez’s death is a reminder of the job’s inherent dangers.

“Our men and women deal with the constant presence of the unknown,” he said, citing drug traffickers, human smugglers, “transnational criminal organizations and the dangerous terrain.”

The agency's Big Bend sector, a mountainous region, accounted for about 1 percent of the more than 61,000 apprehensions its agents made along the Southwest border between October 2016 and May.

“Contrary to public opinion and the views of President Trump, the U.S. side of the Mexican-American border is quite safe,” said Howard Campbell, a border anthropologist at the University of Texas at El Paso. “U.S. border cities are generally safer than their counterparts in the interior of the country. Incidents of violence are not common, and undocumented immigrants are more likely to be the victims of crime than the perpetrators of it.”